Reusable, or hard, baby bottles with attached flexible nipples are commonly used to feed babies formula, water, and other liquids. One limitation associated with such bottles is the tendency of a vacuum to develop within the bottle during feeding. This can occur if, as the baby sucks liquid through the nipple, the liquid is not immediately replaced by air. This creates a pressure differential between the outside environment and the inside of the bottle causing the vacuum.
The vacuum created can, amongst other things, cause the nipple to collapse or cause the feeding portion, or tip, of the nipple to invert and be drawn toward the inside of the bottle. This prevents proper feeding, and may require loosening of the nipple and subsequent reassembly of the bottle. Furthermore, the vacuum created within the bottle can make feeding more difficult, requiring a greater sucking effort which may cause the baby to ingest more air.
The feed opening in the nipple may allow air into the bottle. However, it is insufficient to alleviate the vacuum formed in the bottle since that opening is typically occluded by fluid and the baby's mouth thereby preventing the flow of air therethrough into the bottle.
To address this problem of vacuum creation various pressure equalizing valves have been developed. Most valves require complex valving structures that are expensive to manufacture, difficult to clean, and that can present a potential danger to babies if disassembled and swallowed. Certain prior art nipples employ holes or vents formed in the nipple to allow air pressure to equalize. This structure is not effective as a valve since it does not prevent fluid leakage out through the holes, and does not control the inflow of air.
U.K. Pat. No. 1,432,798 to Yamauchi provides a valve that addresses some of these problems. This pierced-through valve has a small, dome-shape cavity in the nipple flange having stepped cross section. The outer, lower portion or dome-shape of the cavity is wider than the inner portion, and projects below the periphery of the flange. A slit, X-, Y- or I-shape, is then cut into the lower portion of the cavity to form an air passage. An annular rib is formed about the underside of the flange to support and protect the cavity.
U.S. Pat. No. 5,474,028 to Larson provides a check valve in a nipple for feeding animals. This valve is somewhat similar to the valve in the Yamauchi patent in that it projects below the flange. It is also formed in the upper surface of the flange. A supporting annulus or ring is built into the lower face of the nipple flange about the valve to support and protect it.